This invention relates to a solder reflow apparatus which is used for soldering of printed circuit boards using solder cream.
In general, solder reflow apparatuses can be classified as radiation types and hot gas types. A radiation-type reflow apparatus is one in which a large number of panel heaters are disposed in the upper and lower portions of a tunnel, and printed circuit boards are heated by the heat radiated from the panel heaters. The inside of the apparatus can be heated to a suitable temperature for soldering by controlling the current to be supplied to the panel heaters. Recently, panel heaters which emit far infrared radiation have come to be used in reflow apparatuss. Far infrared radiation heaters are efficient at heating objects and are said to be suitable for the soldering of printed circuit boards in solder reflow apparatuses. However, far infrared radiation has the drawback that as printed circuit boards and electronic components become ever thinner and more sensitive to heat, it becomes easier for even slight overheating by the radiation to produce thermal damage. Another problem with radiation-type reflow apparatuses relates to the fact that panel heaters which emit far infrared radiation are made of electrically resistive materials and are heated up by the application of electric current. The temperature of the material which surrounds the heater does not immediately change in response to changes in the heater current. Namely, even if the current to a panel heater is cut off when the heated portion reaches a prescribed temperature, the temperature of the material surrounding the heater may continue to rise for a short while. On the other hand, if the temperature of the heated portion is below a prescribed temperature and current is passed through the heater, the temperature of the material surrounding the heater may be falling and will not immediately start to rise.
A solder reflow apparatus of the hot gas type is one in which hot gas is always circulating past a heater, so there is the advantage that the heating temperature can always be maintained constant. For this reason, in recent years, hot gas reflow furnaces of the tunnel type have come to be much used.
Due to recent advances in the technology of mounting components on printed circuit boards, devices which formerly had to be mounted on separate printed circuit boards can now be mounted on a single printed circuit board. It has also become possible to mount electronic parts such as power transistors on printed circuit boards. However, the packaging density of such printed circuit boards tends to be nonuniform over the area thereof, and this fact along with the presence of large components results in the heat capacity locally varying over the printed circuit board. If such printed circuit boards are soldered using far infrared radiation, the temperature of portions having a high heat capacity will not rise as much as other portions, causing incomplete melting of the solder and poor electrical connections. On the other hand, in those portions of the printed circuit board having a low heat capacity, the temperature will rise too high, causing scorching of the printed circuit board and heat damage to electronic parts.
It has been found that uneven heating of a printed circuit board can be prevented by passing hot gas through a tunnel so as to achieve a uniform temperature within the tunnel. Various devices have been proposed for passing a hot gas through tunnels for this purpose. See, for example, Japanese Laid-Open Utility Model Application No. 59-61567, Japanese Laid-Open Patent Application No. 59-220282, and Japanese Laid-Open Patent Application No. 61-141199. However, each of these devices has various problems.
The solder reflow apparatus disclosed in Japanese Laid-Open Utility Model Application No. 59-61567 has a fan mounted on the upper portion of a tunnel, so hot gas is blown only downwards, and there is no effect from the hot gas except in the region immediately below the fan. Furthermore, as the hot gas circulates only in the vertical direction of the tunnel, it is difficult to adjust the temperature merely by adjusting the rotational speed of the fan.
Japanese Laid-Open Patent Application No. 59-220282 discloses a solder reflow apparatus in which a gaseous heating medium is introduced from the outside of the furnace and is preheated by an auxiliary heater or the like. In that device, it is possible to control the temperature of the hot gaseous medium which flows through the tunnel. However, after passing through the tunnel, the gaseous heating medium is discharged to the outside of the tunnel through a discharge port. Accordingly, this solder reflow apparatus is expensive to manufacture, and it is uneconomical to operate because electricity is used to heat the gaseous heating medium and because the gaseous heating medium is discarded. Furthermore, if the gaseous heating medium is discharged into a room, the room becomes filled with fumes which smell of flux and pose an air pollution problem.
Japanese Laid-Open Patent Application No. 61-141199 discloses a solder reflow apparatus in which air enters through a lower air intake port and is heated by a heater. The hot air is discharged after it has been used for heating, so that invention suffers from the same problems of poor economy and of causing air pollution as does Japanese Laid-Open Patent Application No. 50-220282.
Japanese Laid-Open Patent Application No. 63-215371 corresponding to U.S. Pat. No. 4,771,929 discloses circulation of part of the hot gas withdrawn from the furnace.